The electron density map of the 3D crystal of Photosystem II from Thermosynechococcus vulcanus with a 1.9 angstrom resolution (PDB: 3ARC) exhibits, in the two monomers in the asymmetric unit cell, an, until now, unidentified and uninterpreted strong difference in electron density centered at a distance of around 1.5 angstrom from the nitrogen N delta of the imidazole ring of D2-His336. By MALDI-TOF/MS upon tryptic digestion, it is shown that similar to 20-30% of the fragments containing the D2-His336 residue of Photosystem II from both Thermosynechococcus vulcanus and Thermosynechococcus elongatus bear an extra mass of +16 Da. Such an extra mass likely corresponds to an unprecedented post-translational or chemical hydroxyl modification of histidine.